| Summary: | 碩士 === 國立東華大學 === 光電工程研究所 === 96 === In this thesis, the optical properties of several wurtzite AlGaN/GaN heterostructures have been characterized by photoluminescence(PL), thermo-reflectance(TR) and contactless electroreflectance(CER) measurements. Because the special polarization effects that dissimilar to the groups of GaAs heterostructrres, the AlGaN/GaN heterostructures generate automatically the two-dimentional electron gas(2DEG) at the interface of AlGaN and GaN without modulation doping.
From PL measurements, we can observe the A(heavy hole state), B(light hole state) and C(crystal-field split band) excitonic transitions, which are due to the crystal field effects and the spin-orbital interaction at Γ of the central Brilliouin zone that results in the splitting of valence bands, Γ9, upper Γ7 and lower Γ7.
We can also observe the direct transitions of GaN and AlGaN. In different samples, especially the doped and the undoped GaN layer or AlGaN layer, the full spectral width at half maximum(FWHM) are influnced by the doping concentration, if a sample is undoped, its FWHM would be narrower than a doped one, in addition, the peak is shaper.
By using the Bose-Einstein equation, we can fit the temperature-dependent relationship of transition energies. Through the fitting result, we can see that the energy position would shift to a higher place as the temperature goes down, because of the phonons and the electrons(excitons) interaction. In other words, it’s a blue-shift appearance.
Another part, in the TR spectrum, we can discover the below band-edge interference in the layer of each sample, the oscillation strength grows stronger when the energy range is near to the band gap, and the period is shorter at the same time.
It’s special to discover that at the room temperature, the transitions of the AlGaN layer, is not visible in the PL spectrum while they are present at 300K. The TR spectrum also show the blue-shift appearance with the decrease of temperatures.
FKOs are observed in the CER spectrum, the spectral oscillations appears higher than the energy gap. By using the linear regression estimate, we can obtain the nth extreme of FKOs, if we take the band gap and the effective masses of electrons and holes, another important information will be find: build-in electric field.
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